For the purposes of the ensuing discussion, the following definitions will be in effect. A contact adhesive refers to an adhesive that is applied to at least one of two substrates to be bonded and allowed to dry before mating the two substrates under pressure. An adhesive is considered to be sprayable when it can be applied to a substrate under pressure (whether externally as in a pressure pot arrangement or internally as in a canister) without the adhesive emulsion exhibiting shear instability, edge banding, excessive dripping, overspraying, pulsing or the like. Good green strength is characterized by quick grab or adhesion of the two bonding substrates to each other followed by almost immediate development of high bond strength.
Both solvent- and water-based contact adhesives using polychloroprene are well-known in the art. While solvent-based polychloroprene contact adhesives have the advantage of quicker drying time, insurance and air quality regulations are making water-based contact adhesives increasingly attractive in the marketplace.
Polychloroprene-based contact adhesive properties depend on polychloroprene functionality, crystallinity and gel content. The presence of polar functional groups in the rubber chain provides reactivity with substrate surfaces (especially metals), reactivity with crosslinking agents such as magnesium and zinc oxides to improve heat resistance and green strength and rheology that allows sprayability. As crystallinity increases, there is a corresponding increase in tensile strength, wet contactability, water resistance and room temperature strength. High gel content improves cohesive strength, heat resistance and tensile strength of the adhesive.
Formulators of polychloroprene-based adhesives have devoted extensive effort to optimizing adhesive properties by systematically varying functionality, crystallinity and gel content. Numerous patents have been issued disclosing additional novel approaches to improving and optimizing bond strength, green strength, sprayability and temperature resistance.
Patel (U.S. Pat. No. 6,440,259) discloses the use of a one-part storage-stable water-based contact adhesive with an internal coagulant that develops “enough strength for demanding applications within a short period of time.”
Addition of a tricyclic diterpenecarboxylic acid to a polychloroprene dispersion is reported (Musch, et al, U.S. Pat. No. 6,767,947) to substantially improve heat resistance.
The use of carboxylated polychloroprenes is the easiest and most obvious way to combine functionality, crystallinity and gel content into one polymer system. Pereira et al (U.S. Pat. No. 5,476,896) describe the use of carboxylated polychloroprene (elastomeric properties) and carboxylated ethylene-vinyl acetate (toughness) dispersions in combination with organic crosslinkers to more closely match application characteristics of solvent-based contact adhesives. Carboxylated polychloroprenes for adhesive systems are reported by Sato et al (JP 06256738A2 and JP 06287360A2) to give high shear strength. Masuko (JP 03076734A2) uses a poly(chloroprene-methacrylic acid) polymer for its vibration insulation properties. Polychloroprenes, however, prefer an alkaline environment; the presence of acidity in any form leads to stability and shelf life issues.
Another approach involves incorporation of other carboxylated polymers as grafts onto the polychloroprene rubber base. Carboxylated styrene-butadiene grafts onto polychloroprene are reported by Lima et al (EP Application 1607420A1) to impart good tacking capacity and high heat resistance to the resulting adhesive. Toyo Soda Mfg. Co., Ltd., Japan (JP 59210917A2 and JP 58089602A2) discloses good initial bonding strength from latexes formed by grafting chloroprene onto copolymers with acid-containing monomers. A vulcanization process is reported by Suefuji et al (JP 2002/257199A2) to be useful in preparing polychloroprene rubber power transmission belts. Grafting and vulcanization processes are not necessarily robust in practice and do not always yield reproducible polymer properties.
Other inventors have used polymer blends to adjust adhesive properties. Jaffari et al (EP Application 0517983A1) disclose using a mixture of styrene-butadiene rubber latex (SBR), polychloroprene latex and an ammonium salt of an acrylic polymer as a water-resistant maskant. A high shear strength flooring adhesive prepared by blending polyurethanes with a variety of latexes including polychloroprenes and carboxylated SBR is reported by Wood, et al (U.S. Pat. No. 5,721,302). Rayner (U.S. Pat. No. 6,310,125) uses a blend of high modulus polyurethane, polychloroprene and an acrylic ester copolymer to prepare adhesives with excellent heat resistance. Horiuchi et al (JP 2002/121516A2) blend poly(ethylene oxide), poly(N-vinylacetamide) or poly(acrylic acid) with polychloroprene to obtain good adhesion to polyurethane foam. Addition of poly(acrylic acid) sodium salt is reported by Ono et al (JP 05320600A2) to improve spray coatability of polychloroprene adhesives. Not all carboxylated polymers are compatible with the polychloroprene. Phase separation of the blended polymers can lead to inferior adhesive properties.
Carboxylated polymers are of particular interest in these systems since they can be crosslinked using organic (see Pereira et al, U.S. Pat. No. 5,476,896 cited above) or metal oxide crosslinkers. The use of metal oxide crosslinkers in a carboxylated polychloroprene is reported by Lew et al (WO 2004/044037A1) to give improved tensile strength and non-staining latex gloves.
Acrylic waterborne contact adhesives are known (Shah, U.S. Pat. No. 5,543,455) but they are expensive, lack the strength inherent to the crystallinity in polychloroprene and resistance to high temperature. Taga et al (JP 04023877A2) has even used a mixture of polychloroprene and carboxylated acrylic emulsions but these must be applied separately because of stability issues.
None of the above expedients, other than carboxylated polychloroprene, give a water-based contact adhesive with the combination of good sprayability, green strength, heat resistance and long-term bond strength.